Mobile telephony systems, in which user equipment such as mobile handsets communicate via wireless links to a network of base stations connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue modulation has been superseded by second generation digital systems, which are themselves currently being superseded by third generation digital systems such as UMTS and CDMA. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposal by the Third Generation Partnership Project of the so-called Long Term Evolution system, often simply called LTE, which offers potentially greater capacity still, by the use of wider frequency bands, spectrally efficient modulation techniques and potentially also the exploitation of spatially diverse propagation paths to increase capacity (Multiple In Multiple Out).
Distinct from mobile telephony systems, wireless access systems have also undergone development, initially aimed at providing the “last mile” (or thereabouts) connection between user equipment at a subscriber's premises and the public switched telephone network (PSTN). Such user equipment is typically a terminal to which a telephone or computer is connected, and with early systems there was no provision for mobility or roaming of the user equipment between base stations. However, the WiMax standard (IEEE 802.16) has provided a means for such terminals to connect to the PSTN via high data rate wireless access systems.
Whilst WiMax and LTE have evolved via different routes, both can be characterised as high capacity wireless data systems that serve a similar purpose, typically using similar technology, and in addition both are deployed in a cellular layout as cellular wireless systems. Typically such cellular wireless systems comprise user equipment such as mobile telephony handsets or wireless terminals, a number of base stations, each potentially communicating over what are termed access links with many user equipments located in a coverage area known as a cell, and a two way connection, known as backhaul, between each base station and a telecommunications network such as the PSTN.
As the data capacity of cellular wireless systems increases, this in turn places increasing demands on the capacity of the backhaul, since this is the connection that has to convey the wireless-originating traffic to its destination, often in an entirely different network. For earlier generations of cellular wireless systems, the backhaul has been provided by one or more connections leased from another telecommunications operator (where such a connection exists near to the base station); however, in view of the increasing data rates, the number of leased lines that is required is also increasing. Consequently, the operational expense associated with adopting multiple leased lines has also increased, making this a potentially expensive option for high capacity systems.
As an alternative to leased lines, dedicated backhaul links can be provided by a variety of methods including microwave links or optical fibre links. However each of these methods of backhaul has associated costs. Dedicated fibre links can be expensive in terms of capital expense due mainly to the cost of the civil works in installation, and this problem is especially acute in urban areas. Microwave links also involve the capital expense of equipment and require expert installation due to narrow beam widths leading to the requirement for precise alignment of antennas.
As an alternative to the provision of a dedicated backhaul link for each individual base station, it is possible to use the radio resource of the cellular wireless system to relay backhaul traffic from one base station to another. Typically, the base station using the cellular radio resource for backhaul is a small low power base station with an omnidirectional antenna known as a relay node. Such a system can be used to extend the area of cellular wireless coverage beyond the area of coverage of conventional base stations that are already equipped with a dedicated backhaul.
FIG. 1 shows a conventional wireless cellular network; in this example, base stations 2a . . . 2g are connected by microwave links 4a . . . 4c to a microwave station 6 and thence to a telecommunications network 8.
FIG. 2 shows a conventional relay node operating within a cellular wireless network; the operation may for example be in accordance with IEEE 802.16j. A user equipment 12b is in communication with a relay node base station 10. As the relay node 10 is not provided with a backhaul link separate from the cellular wireless resource, the relay node is allocated radio resource timeslots for use relaying backhaul data to and from the adjacent base station 2 which is itself connected by microwave link to a microwave station 6 and thence to a telecommunications network 8 such as the public switched telephone network. A user equipment 12a is shown in communication with the base station 2.
FIG. 3 shows a conventional time frame structure allocating timeslots alternately to access 14a . . . 14d and backhaul 16a . . . 16c. Typically, all of the access payload data will be relayed by the backhaul link; if the spectral efficiency of the backhaul and access links is the same, then the access and backhaul timeslots will occupy approximately equal amounts of time. There may be a significant reduction in capacity available in the access links to the user equipment due to the need to reserve timeslots for backhaul.
Typically a relay node will be deployed within a wireless cellular network in which many of the base stations are equipped with dedicated backhaul connections such as microwave links and are typically not associated with other relay nodes. The allocation of timeslots for backhaul is localized around the relay node, so that, unlike the timeslot allocation depicted in FIG. 3, base stations which are not associated with relay nodes are typically not allocated backhaul timeslots at all. The allocation of timeslots to such base stations is illustrated in FIG. 4a, and accordingly indicates a typical time frame structure allocated to a base station with dedicated backhaul but which is not associated with a relay node. FIG. 4b, however, indicates a typical time frame structure that might be allocated when the base station is associated with relay nodes such that access payload data is relayed by the backhaul link (base stations and relay nodes having this relationship can be considered to be part of a relay zone); the operation may for example be in accordance with IEEE 802.16j.
The time frames shown in FIG. 4a and FIG. 4b repeat in time, so that for example the relay zone time frame as illustrated in FIG. 4b represents a section of the alternating backhaul and access timeslot sequence shown in FIG. 3.
In the case of the time frame structure of FIG. 4a for a base station with dedicated backhaul, the frame is divided into a downlink section 18, 20, 22 in which the base station transmits, and an uplink section 24 in which user equipments transmit. The division 32 between downlink and uplink timeslots is typically constant throughout a wireless network within an area of contiguous wireless coverage. This is to prevent the situation arising in which a base station is transmitting at the same time as another base station is receiving, since this could cause interference. As base stations are typically mounted on towers and transmit at high power, the interference could be propagated for a considerable distance.
In the case of the time frame structure of FIG. 4b for the relay zone, the frame is divided as before into a downlink section 18, 20, 22 in which the base station transmits to user equipments and an uplink section 24 in which user equipments transmit to the base station. A backhaul timeslot 16 is inserted between the access downlink 22 and access uplink 24 sections. The backhaul timeslot 16 is divided into a timeslot 26, 28 in which the base station transmits to a relay node and a timeslot 30 in which a relay node transmits to a base station. As a result, sections indicated by reference numerals 18, 20, 22, 26 and 28 represent timeslots when the base station is transmitting, and reference numerals 30 and 24 represent timeslots when the base station is receiving. It is important to note that conventionally the division 32 between the timeslots when the base station is transmitting and the timeslots when the base station is receiving is the same for base stations with dedicated backhaul as for base stations in a relay zone within an area of contiguous wireless coverage.
The conventional allocation of timeslots to backhaul as illustrated in FIGS. 4a and 4b has the benefit that it approximately maintains the proportion of time allocated to uplink and downlink access timeslots, and that the system can re-use the existing mechanisms that control the timing of the transition between transmission and reception modes. However, in the timeslot indicated by reference numeral 30 in FIG. 4b, the relay node is transmitting in part of the timeslot 24 in which base stations with conventional, i.e. dedicated, backhaul are receiving. If the relay node were mounted on a tower or were operating at high power, this would potentially cause interference to the access uplink of base stations with dedicated backhaul as has already been mentioned. This places restrictions on the operating power and siting of relay nodes or can restrict the performance of the network
Hence it can be seen that backhaul links for high capacity cellular wireless systems can present a significant expense; to mitigate this, the cellular wireless resource can be used to relay backhaul links from one base station to another, but when employed in conventional arrangements, this incurs significant limitations to data capacity and restrictions on the positioning and transmission power of base stations.